5,491 research outputs found
Fermi volume evolution and crystal field excitations in heavy-fermion compounds probed by time-domain terahertz spectroscopy
We measure the quasiparticle weight in the heavy-fermion compound
CeCuAu () by time-resolved THz spectroscopy for
temperatures from 2 up to 300\,K. This method distinguishes contributions from
the heavy Kondo band and from the crystal-electric-field satellite bands by
different THz response delay times. We find that the formation of heavy bands
is controlled by an exponentially enhanced, high-energy Kondo scale once the
crystal-electric-field states become thermally occupied. We corroborate these
observations by temperature-dependent dynamical mean-field calculations for the
multi-orbital Anderson lattice model and discuss consequences for quantum
critical scenarios.Comment: Published version, 6 pages (including references), 5 figures,
Supplemental Material (2 pages) adde
Itinerant quantum critical point with frustration and non-Fermi-liquid
Employing the self-learning quantum Monte Carlo algorithm, we investigate the
frustrated transverse-field triangle-lattice Ising model coupled to a Fermi
surface. Without fermions, the spin degrees of freedom undergoes a second-order
quantum phase transition between paramagnetic and clock-ordered phases. This
quantum critical point (QCP) has an emergent U(1) symmetry and thus belongs to
the (2+1)D XY universality class. In the presence of fermions, spin
fluctuations introduce effective interactions among fermions and distort the
bare Fermi surface towards an interacting one with hot spots and Fermi pockets.
Near the QCP, non-Fermi-liquid behavior are observed at the hot spots, and the
QCP is rendered into a different universality with Hertz-Millis type exponents.
The detailed properties of this QCP and possibly related experimental systems
are also discussed.Comment: 9 pages, 8 figure
Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz
In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al.
reported the development of a terahertz quantum cascade laser operating at 3.4
THz or 14.2 meV. We have calculated and analyzed the gain spectra of the
quantum cascade structure described in their work, and in addition to gain at
the reported lasing energy of ~= 14 meV, we have discovered substantial gain at
a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue
for the development of a terahertz laser at this lower energy, or of a
two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200
Optical Transistor for an Amplification of Radiation in a Broadband THz Domain
We propose a new type of optical transistor for a broadband amplification of
THz radiation. It is made of a graphene--superconductor hybrid, where electrons
and Cooper pairs couple by Coulomb forces. The transistor operates via the
propagation of surface plasmons in both layers, and the origin of amplification
is the quantum capacitance of graphene. It leads to THz waves amplification,
the negative power absorption, and as a result, the system yields positive
gain, and the hybrid acts like an optical transistor, operating with the
terahertz light. It can, in principle, amplify even a whole spectrum of chaotic
signals (or noise), that is required for numerous biological applications.Comment: 7 pages, 3 figure
A multi-purpose method for analysis of spur gear tooth loading
A large digitized approach was developed for the static and dynamic load analysis of spur gearing. An iterative procedure was used to calculate directly the "variable-variable" gear mesh stiffness as a function of transmitted load, gear tooth profile errors, gear tooth deflections and gear hub torsional deformation, and position of contacting profile points. The developed approach can be used to analyze the loads, Hertz stresses, and PV for the normal and high contrast ratio gearing, presently the modeling is limited to the condition that for a given gear all teeth have identical spacing and profiles (with or without surface imperfections). Certain types of simulated sinusoidal profile errors and pitting can cause interruptions of the gear mesh stiffness function and, thus, increase the dynamic loads in spur gearing. In addition, a finite element stress and mesh subprogram was developed for future introduction into the main program for calculating the gear tooth bending stresses under dynamic loads
First-principles Analysis of Photo-current in Graphene PN Junctions
We report a first principles investigation of photocurrent generation by
graphene PN junctions. The junctions are formed by either chemically doping
with nitrogen and boron atoms, or by controlling gate voltages. Non-equilibrium
Green's function (NEGF) formalism combined with density functional theory (DFT)
is applied to calculate the photo-response function. The graphene PN junctions
show a broad band photo-response including the terahertz range. The dependence
of the response on the angle between the light polarization vector and the PN
interface is determined. Its variation against photon energy is
calculated in the visible range. The essential properties of chemically doped
and gate-controlled PN junctions are similar, but the former shows fingerprints
of dopant distribution.Comment: 7 pages, 6 figure
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